In shallow arctic waters off the northern coasts of Alaska and Canada, man-made islands have been built to provide support for oil and gas exploration and production facilities. The islands typically have been made from fill material such as gravel placed on the sea floor at the offshore site. Such islands, however, only have practical application in shallow waters because the amounts of fill material required and associated construction costs increase greatly with small increases in water depth. Moreover, satisfactory fill material is a relatively scarce commodity in many arctic regions requiring transportation of the same over great distances. Construction of such islands is even further hindered by the relatively short construction season when the installation site is free or relatively free of ice. Of course, there also are environmental considerations in view of the relative permanence of the islands in relation to mobile platform structures that have been used in non-arctic regions for exploratory drilling.
Mobile platform structures heretofore used in non-arctic regions generally fall into two categories consisting of those intended for deep water installation and others intended for relatively shallow water installation. For example, one proposed platform structure intended for deep water installation, as at depths on the order of 150 and 300 feet, is disclosed in U.S. Pat. No. 3,277,653. Such structure includes an elevated platform supported by a tripod arrangement of three vertical legs that may be separately driven into the sea floor foundation as through a layer of alluvial soil or soft mud and into the underlying sands to obtain desired load bearing capacity. The shallow water platforms are typified by barge-like structures and some are known to employ spud piles for anchoring purposes while others have employed fixed skirts. Notwithstanding any differences between these two categories of structures, both commonly have provision for floating the structure for deployment and redeployment at installation sites.
These non-arctic structures, however, are not designed to withstand the large ice loads encountered in arctic offshore regions. In addition to large compressive or crushing ice loads, a structure can be subjected to gigantic lateral forces due to ice movement during the winter and ice floe impacts during the summer. Loading severities increase dramatically in moving from shallow waters in which shore-fast ice dominates to the deeper waters of the transition zone that interfaces with the Polar Pack. In the transition zone or relatively open water areas, ice sheet formations move considerably and thus impose very high lateral displacement forces on any structure in their paths. Obviously, any such structure used for exploratory drilling purposes must remain laterally fixed against such forces to maintain vertical alignment of the structure with the well or wells being drilled. Those structures employing skirts also are unacceptable because of draft requirements imposed by the arctic waters off the northern coast of Alaska, specifically Point Barrow, and because of the inability of the typical shallow water arctic platform to force the skirts into stronger soils.
A few mobile arctic platform structures have been proposed. One proposed structure is disclosed in U.S. Pat. No. 3,793,840 and includes a vertically narrow, barge-like base which may be ballasted onto the sea floor. Supported on the base is a conical shell provided with an upwardly extending column which receives a vertically positionable deck. The base is anchored against lateral forces primarily by a caisson which is longitudinally movable inside the base for embedment in the sea floor foundation and secondarily or optionally by a plurality of relatively short spud piles driven through openings in the base and into the sea floor foundation. When redeployment of the structure is desired, the structure is separated from the caisson and presumably the spud piles by deballasting the base, the structure then rising clear of the caisson and piles for refloating to another installation site.
Another proposed structure is disclosed in U.S. Pat. No. 4,245,929 and includes a multi-angle conical base which supports a work platform. The base is provided with ballast chambers for partial submergence onto and into the sea floor foundation. For resistance against lateral ice loads, the structure relies significantly on the ice breaking ability of its conical base. For unusually severe ice conditions, piles may be driven through guides in the base and into the sea floor foundation to assist in holding the structure in place, such piles being detached from the structure prior to its being floated to a new drilling site.
A common feature of both of these mobile arctic structures is their provision of a broad base support which is considerably larger or at least about twice as large as the supported working deck in horizontal dimension. Accordingly, these types of structures, when outfitted with a typical working desk, would be quite massive and have extremely wide bases, and additionally would result in increased seismic forces and probably increased ice forces. In addition, the pilings or spud piles associated with either structure are not surface accessible for ease in driving the same into the sea floor foundation from the structure itself. Presumably, another vessel is required for purposes of driving the piles located laterally remote of any above water support or deck on the structure. Also, the spud piles are treated as an expendable but costly commodity inasmuch as they are left behind upon relocation of the structure. It further is noted that the structure disclosed in U.S. Pat. No. 3,793,840 has the further drawback that any lateral shifting of the structure due to extreme loads necessarily will cause the caisson to shift thus presenting the possibility of damage to the therein located well heads and associated equipment.